The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications

Abstract

Coesite-bearing eclogitic rocks from the Pan-African belt in northern Mali were dated by multichronological methods (Rb–Sr, Sm–Nd and Ar–Ar). Rb–Sr and Sm–Nd isotope analyses on whole rock and mineral separates of an omphacite–kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 Ma. This is interpreted as the time of the eclogitisation, which represents the oldest ultrahigh-pressure (UHP) metamorphic event so far recorded in the continental crust. The nearly identical Rb–Sr and Sm–Nd ages suggest very rapid exhumation as observed in many Phanerozoic UHP rocks. 40Ar– 39Ar dating of phengites from two UHP micaschists gave much older ages of 1045 and 760 Ma. Phengite from a quartzite collected from the same site yielded a plateau date of 623±3 Ma, which is in agreement with the Rb–Sr and Sm–Nd dates. The two 40Ar– 39Ar ages, older than 625 Ma, testify once more the recurrent problem of excess Ar in ultrahigh-pressure phengites. The occurrence of the Mali UHP eclogites indicates that by the late Precambrian, the Earth has cooled enough to sustain the formation and preservation of deeply subducted UHP metamorphic rocks. The new ages of ca. 620 Ma also constrain the amalgamation in NW Gondwana to have occurred in the latest Proterozoic, but not in the Middle Cambrian. This is supported by several recent chronological data for the southern segment of the major suture zone in the Dahomeyide orogenic belt.